Chloride is the most abundant extracellular anion. In general, chloride is affected by the same conditions that affect sodium (the most abundant extracellular cation) and in roughly the same degree. Thus, in the great majority of cases, serum chloride values change in the same direction as serum sodium values (except in a few conditions such as the hyperchloremic alkalosis of prolonged vomiting). For example, if the serum sodium concentration is low, one can usually predict that the chloride concentration will also be low (or at the lower edge of the reference range). To confirm this I did a study comparing sodium and chloride values in 649 consecutive patients. There were 37 discrepancies (5.7%) in the expected relationship between the sodium and chloride values. On repeat testing of the discrepant specimens, 21 of the 37 discrepancies were resolved, leaving only 16 (2.5%). Of these, 6 (1%) could be classified as minor in degree and 10 (1.5%) as significant. Thus, in 649 patients only 1.5% had a significant divergence between serum sodium and chloride values.